:: headcanon

Headcanon is basically what is canon to the character, but not in the actual story. Therefore, what you see here are my interpretations at this version of Sonic. 

• The injection was never really explained. For this role-play account, it’s a certain serum that bonded into his blood stream. The bonding process hurt due to the body trying to fight it’s multiplication, as well as the body refusing to be taken over. The taken over part was Sonic’s own will, and it unfortunately didn’t work. 
• The finale movie implies that his memories were lost shortly after the take over. The only thing he remembered was his name; and it even wiped subconscious feelings for his friends, such as his love for Sally. 
• Due to not knowing anything, Sonic occasionally has fits of over-thinking his identity, and if he gets sent into it just enough, the serum will retaliate over such thoughts, ESPECIALLY if he tries to consider the story of Sonic the Hero to be true.
• Even though what created this Sonic is a serum, therefore a liquid, a chemical inside it was a robotization formula of some sort. This shows in his eyes, which can glow alike Metal Sonic. (Even though there isn’t a Metal Sonic in this universe.) 
• Despite being taken over, this Sonic still has his weaker moments where he can’t say no to helping others. Unfortunately, this won’t cause heroic deeds. It’ll cause sympathy, and a loving want to make a hurt little kid forget all their troubles. But…it’s rather small. And hard to witness. 

To be expanded upon as we go on.